The parameters of high frequency transistors with implanted arsenic emitter and boron base were found to change significantly on annealing at temperatures between 600 ~ and 900~ with a maximum change at about 700~ The change can be interpreted as a decrease of boron concentration in the base and of the donor concentration gradient at the emitter-base junction. Determination of the concentration profiles by secondary ion mass spectrometry and by Rutherford backscattering spectrometry (RBS) showed that these decreases were due to enhanced diffusion of both arsenic and boron. Electron microscopy showed precipitation which was probably preceded by arsenic clustering, in agreement with RBS channeling results. These showed that the substitutional fraction of arsenic, which was between 95% and 98%, decreased by a few percent on annealing at 700~ The enhanced diffusion is thought to be caused by vacancy generation by either one of two precipitation-induced processes: dissociation of As-vacancy complexes or growth of interstitial defects.Implanted arsenic is now widely used as a donortype impurity in silicon. For example, bipolar transistors are made with implanted boron base and arsenic emitter. After implantation the As and B are electrically activated by an appropriate heat-treatment around 1000~ In a process which involved a subsequent annealing at 650~ we found that the parameters of high frequency transistors with shallow As emitter (0.2 #m) and B base (0.35 #m) were drastically affected by this annealing. The current gain, for example, was greatly increased.The effects of low temperature heat-treatments on Si with As diffused into it have been studied earlier.Osvenskii et al. (1) found interstitial dislocation loops by transmission electron microscopy on As-doped Si crystals which had been annealed at 800~ in vacuum after quenching from 1250~The loops were attributed to As precipitation. Schwenker et al. (2) found for As diffused at 1050 ~ and ll00~ to a depth of 6-9 #m into Si that heat-treatment between 500 o and 970~ in argon caused the sheet resistance to increase. Around 750~ this increase was maximum, being a factor of 2 at a surface concentration of 1.2 • 1021 cm -3 and less at lower concentrations. Transmission electron microscopy showed dark spots and interstitial hexagonal Frank dislocation loops. The effects on sheet resistance were ascribed to As clustering or vacancy-complex formation. An increase in sheet resistance due to heat-treatment between 500 ~ and 800~in vacuum was also found by Miyamoto et al. (3) who observed an increase of the lattice constant, which was supposed to be due to SiAS formation. Haskell et aI.(4) studied Rutherford backscattering on As in Si and found from channeling that 90% to 95% of the As was on lattice sites. Heat-treatment between 650 ~ and 900~ in argon increased the aligned As yield, showing As to be displaced from substitutional lattice sites.Shibayama et al. (5) observed an enhancement of both B and As diffusion in Si for As surface concentrations above about 1020 cm ...
